Level II scour analysis for Bridge 33 (BRIDTH00050033) on Town Highway 5, crossing the North Branch Ottauquechee River, Bridgewater, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
BRIDTH00050033 on town highway 5 crossing the North Branch Ottauquechee River,
Bridgewater, Vermont (figures 1–8). A Level II study is a basic engineering analysis of the
site, including a quantitative analysis of stream stability and scour (U.S. Department of
Transportation, 1993). Results of a Level I scour investigation also are included in
Appendix E of this report. A Level I investigation provides a qualitative geomorphic
characterization of the study site. Information on the bridge, gleaned from Vermont Agency
of Transportation (VTAOT) files, was compiled prior to conducting Level I and Level II
analyses and is found in Appendix D.

The site is in the Green Mountain section of the New England physiographic province of
central Vermont in the town of Bridgewater. The 5.01-mi²
drainage area is in a
predominantly rural and forested basin. In the vicinity of the study site, the downstream
banks are forested and the upstream banks have dense woody brush; the upstream right
overbank is an open field.

In the study area, the North Branch Ottauquechee River has an incised, sinuous channel
with a slope of approximately 0.017 ft/ft, an average channel top width of 30 ft and an
average channel depth of 3 ft. The predominant channel bed materials are gravel and cobble
with a median grain size (D₅₀) of 83.2 mm (0.273 ft). The geomorphic assessment at the
time of the Level I and Level II site visit on November 3, 1994, indicated that the reach was
stable. Also at the time of the site visit, there was considerable backwater at the bridge site
due to a three foot tall beaver dam 40 feet downstream. The beaver dam was assumed
destroyed by flood flow and was ignored in the analyses.

The town highway 5 crossing of the North Branch Ottauquechee Riveris a 25-ft-long, onelane bridge consisting of one 23-foot steel-beam span with a timber deck (Vermont Agency
of Transportation, written communication, August 25, 1994). The bridge is supported by
vertical, concrete abutments with wingwalls. The channel is skewed approximately 20
degrees to the opening while the opening-skew-to-roadway is 10 degrees.

A scour hole 1.0 ft deeper than the mean thalweg depth was observed along the right
abutment and upstream right wingwall during the Level I assessment. Scour protection
measures at the site include type-2 stone fill (less than 36 inches diameter) at the ends of all
the wingwalls except the upstream left which has
type-3 stone fill (less than 48 inches diameter). Additional details describing conditions at
the site are included in the Level II Summary and Appendices
D and E.

Scour depths and rock rip-rap sizes were computed using the general guidelines described
in Hydraulic Engineering Circular 18 (Richardson and others, 1993). Total scour at a
highway crossing is comprised of three components: 1) long-term streambed degradation;
2) contraction scour (due to accelerated flow caused by a reduction in flow area at a bridge)
and; 3) local scour (caused by accelerated flow around piers and abutments). Total scour is
the sum of the three components. Equations are available to compute depths for contraction
and local scour and a summary of the results of these computations follows.

Contraction scour for all modelled flows ranged from 0.0 to 0.7 ft. The worst-case
contraction scour occurred at the incipient-overtopping discharge, which was less than the
100-year discharge. Abutment scour ranged from 5.3 to 7.2 ft. The worst-case abutment
scour occurred at the 500-year discharge. Additional information on scour depths and
depths to armoring are included in the section titled “Scour Results”. Scoured-streambed
elevations, based on the calculated scour depths, are presented in tables 1 and 2. A crosssection of the scour computed at the bridge is presented in figure 8. Scour depths were
calculated assuming an infinite depth of erosive material and a homogeneous particle-size
distribution.

It is generally accepted that the Froehlich equation (abutment scour) gives “excessively
conservative estimates of scour depths” (Richardson and others, 1995, p. 47). Usually,
computed scour depths are evaluated in combination with other information including (but
not limited to) historical performance during flood events, the geomorphic stability
assessment, existing scour protection measures, and the results of the hydraulic analyses.
Therefore, scour depths adopted by VTAOT may differ from the computed values
documented herein.